This invention is concerned with the field of nondestructive testing and, more particularly, with the use of ultrasonic waves in nondestructive testing.
Nondestructive testing is a branch of materials science which is concerned with all aspects of quality and uniformity in materials. Ultrasonic techniques have proven to be a useful tool in a variety of nondestructive testing measurement tasks, covering all phases of testing. At the research and development stage, for example, ultrasonics may be utilized to identify material variables. In addition, ultrasonic techniques may advantageously be applied as a quality control measure during production, in process controls designed to ensure the uniformity of a continuously produced product, and as a part of on-site inspections of installed systems. Finally, ultrasonic testing systems are increasingly being applied to examine in-service components to detect failure parameters, such as wear, deterioration, corrosion, etc.
Within the area of ultrasonics, a number of different types of wave energy may be utilized. Longitudinal waves, Rayleigh waves, and shear waves (polarized horizontally, vertically, or angularly) will propagate in an elastic material. In a bounded medium, such as a plate, ultrasonic waves will travel in the form of guided waves, a phenomenon which is analogous to the transmission of microwaves in a waveguide. Horizontally polarized shear waves are one wave type which will propagate in a plate medium. In addition, although vertically polarized shear and longitudinal waves do not satisfy the plate boundary conditions individually, a combination of those two wave types will propagate in a combined form which is known as a Lamb wave. Two families of Lamb waves, antisymmetric and symmetric, can be generated in a plate-type medium.
Whatever wave type is utilized, an essential component in any ultrasonic testing system is a device which will effectively generate the desired ultrasonic waves. This function has frequently been accomplished in the past by piezoelectric transducers, which utilize the piezoelectric effect to convert electrical energy into the mechanical energy of wave motion. Piezoelectric transducers, however, require some form of mechanical contact with the material in which the waves are to be generated, and in many applications this requirement has limited the usefulness of such transducers.
A new family of ultrasonic transducers has recently been developed which offers significant operational advantages over the tranditional piezoelectric systems. These new transducers, known as electromagnetic acoustic transducers (EMATs) do not require any mechanical contact with the wave medium and are capable of operating at elevated temperatures and high speeds. An EMAT employs a coil of wire, which is driven at a dynamic frequency .omega., and a permanent or electromagnet, which is utilized to produce a static magnetic bias field H.sub.o. When such an EMAT is placed near an electrically conductive material, ultrasonic waves are generated in the material as a result of the Lorentz body force EQU F=.mu..sub.o J.sub..omega. .times.H.sub.o
where J.sub..omega. is the dynamic eddy current which is induced in the material by the coil of the EMAT. Furthermore, ultrasonic waves which are propagating in a material can be detected by the same type of transducer through the reciprocal process. The Lorentz mechanism by which such a transducer operates is familiar as the principle upon which the operation of an ordinary electric motor is based.
A variety of EMAT coil and magnet configurations have been developed to couple to particular elastic wave modes or polarizations. One of the most widely used EMAT transducer types, for example, is the meander coil EMAT transducer, examples of which are disclosed in U.S. Pat. Nos. 3,850,028 and 4,048,847, which are incorporated herein by reference. A meander coil transducer can be utilized to excite Rayleigh, Lamb, vertically polarized angle shear, or angle longitudinal ultrasonic waves, depending on the frequency at which the transducer is driven. In addition, when a meander coil transducer is equipped with an electromagnet, it can be adjusted to exert that static magnetic bias strength which provides maximum efficiency in ultrasonic wave generation.
Meander coil transducers have heretofore been employed to generate and detect ultrasonic waves used in testing for the presence of flaws in various materials. The wave types which typically are generated, however, such as Lamb waves, are subject to mode conversion when those waves are reflected from boundaries of the material or from flaws and other discontinuities in the material. Lamb waves, for example, will readily convert from the antisymmetric family to the symmetric family of Lamb waves. Because of this mode conversion effect, it has proven desirable in some applications to employ horizontally polarized shear waves, which are less subject to mode conversion, and thereby avoid spurious reflections and other coherent noise introduced by mode conversion. Horizontally polarized shear waves are more difficult to produce but may be generated and detected by another kind of EMAT, known as a permanent magnet EMAT. Typical examples of permanent magnet EMATs are disclosed in U.S. Pat. No. 4,127,035, which is incorporated herein by reference. Although there is thus available a transducer which is capable of generating and detecting horizontally polarized shear waves, the periodic magnet EMAT exhibits a number of operational disadvantages. When used on ferromagnetic materials, for example, such a transducer operates with a variable efficiency, which can introduce data interpretation problems. Furthermore, the design of the coil of such a transducer is such that a periodic magnet EMAT exhibits relatively high electrical losses relative to other transducer designs, such as the meander coil. Finally, the relatively high impedance of a periodic magnet EMAT makes it difficult to drive such a transducer with solid state circuitry. In addition, it is difficult to produce such an EMAT with an efficient directional radiation pattern, although unidirectional operation is sometimes necessary in particular applications.
Therefore, a need has developed for an improved transducer design which will generate or detect horizontally polarized shear waves.
In addition, a need has developed for a horizontally polarized shear wave transducer in which the magnetic bias may be varied in order to operate the transducer at maximum efficiency.
A need has also developed for a transducer which is capable of generating horizontally polarized shear waves in one direction in order to provide optimum utilization of the propagated wave energy.
Finally, a need has developed in the art for an improved transducer design which will generate horizontally polarized shear waves with a relatively low amount of electrical leakage from the transducer.